In recent years, a decrease in size and weight, an increase in power output and an increase in efficiency have been required in a permanent magnet motor used in a hybrid car, a hard disk drive, or the like. To realize such a decrease in size and weight, an increase in power output and an increase in efficiency in the permanent magnet motor mentioned above, a further improvement in magnetic performance is required of a permanent magnet to be buried in the permanent magnet motor. Meanwhile, as permanent magnet, there have been known ferrite magnets, Sm—Co-based magnets, Nd—Fe—B-based magnets, Sm2Fe17Nx-based magnets or the like. As permanent magnet for permanent magnet motor, there are typically used Nd—Fe—B-based magnets among them due to remarkably high residual magnetic flux density.
As method for manufacturing a permanent magnet, a powder sintering process is generally used. In this powder sintering process, raw material is coarsely milled first and furthermore, is finely milled into magnet powder by a jet mill (dry-milling) method. Thereafter, the magnet powder is put in a mold and pressed to form in a desired shape with magnetic field applied from outside. Then, the magnet powder formed and solidified in the desired shape is sintered at a predetermined temperature (for instance, at a temperature between 800 and 1150 degrees Celsius for the case of Nd—Fe—B-based magnet) for completion.
Further, there are conventionally practiced, when manufacturing permanent magnet, to increase the amount of rare earth elements among the constituent elements contained in the magnet raw material larger than the amount based upon stoichiometric composition (for example, Nd: 26.7 wt %, Fe (electrolytic iron): 72.3 wt %, B: 1.0 wt %) so as to form a phase which is rich in rare earth elements (such as Nd-rich phase) in grain boundaries (hereinafter abbreviated to “rich phase”).
Then, in the permanent magnet, the rich phase has the following features. The rich phase:    (1) has a low melting point (approx. 600 degrees Celsius) and turns into a liquid phase at sintering, contributing to densification of the magnet, which means improvement in magnetization;    (2) can eliminate surface irregularity of the grain boundaries, decreasing nucleation sites of reverse magnetic domain and enhancing coercive force; and    (3) can magnetically insulate the main phase, increasing the coercive force.
Poorly dispersed rich phase in the sintered permanent magnet potentially causes a partial sintering defect and degrade in the magnetic property; therefore it is important to have the rich phase uniformly dispersed in the sintered permanent magnet.